0610 · 18.1Variation

Variation, genes and phenotypes

0:00Imagine growing several pea plants from seeds of the same species. Even if you plant them together, they may not all reach exactly the same height, and their seeds may differ in colour or shape. Why are some differences gradual, while others fall into separate categories? By the end of this video, you will understand these patterns, how to investigate them, and where genetic variation comes from. Variation means differences between individuals of the same species.

0:24That last part matters because comparing a pea plant with an oak tree is comparing different species, not investigating variation within one species. Differences in body length, body mass and blood group can all be examples of variation. However, these differences do not all have the same causes or follow the same pattern. We need three pieces of background knowledge before we begin. Genes are sections of deoxyribonucleic acid, the molecule that carries genetic information, and alleles are different versions of a gene.

0:53A phenotype is an organism’s observable features, such as its body length or seed colour. Observable does not mean visible just by looking. Blood group is a phenotype, even though you need a test to identify it.

Continuous variation

1:05First, consider body length in individuals of the same species. You might measure a short individual, a slightly longer one, and many others with lengths between the shortest and longest. There is no biological rule that places every individual into just two categories, short or long. This is continuous variation. It results in a range of phenotypes between two extremes. Body mass is another example of continuous variation. Individuals can have many different masses, with intermediate values between the lightest and heaviest.

1:35We record measurements to a limited precision, but that does not turn the characteristic into discontinuous variation. A balance displaying whole grams is simply rounding the measurement, and the organisms do not naturally belong to separate whole gram categories. Continuous variation is caused by both genes and the environment. Genes influence how an organism grows, but environmental conditions influence how that growth develops. For a plant, water, mineral availability and light can affect its final size.

2:03For a human, nutrition can affect growth and body mass, so the phenotype is not simply a readout of the genes. Think of genes as contributing to the instructions for growth, while the environment affects the conditions under which those instructions are used. Two plants with similar inherited characteristics may reach different sizes if one receives enough water and the other does not. Equally, plants grown under similar conditions can differ because their genes differ.

2:27Many possible combinations of inherited influences and environmental conditions help produce a range of intermediate phenotypes.

Discontinuous variation

2:35Now compare that with seed shape in peas. For the standard inherited characteristic studied at this level, mature seeds are classified as round or wrinkled. They belong to distinct categories rather than forming a smooth range of intermediate seed shapes. This is discontinuous variation. It results in a limited number of phenotypes with no intermediates. Seed colour in peas is another example, with yellow and green as the categories studied here.

2:59The main human blood groups also show discontinuous variation, with four distinct categories rather than a continuous scale. Someone does not have a blood group halfway between two categories. The important question is whether intermediate phenotypes exist, not whether one category is more common than another. Discontinuous variation is usually caused by genes only. In our examples, inherited alleles determine the category, rather than environmental conditions producing a range of intermediate categories.

3:26Changing your diet does not move your inherited blood group gradually towards another blood group. Notice the word usually. It is more accurate than claiming that every characteristic with separate categories must always be determined only by genes. Also, do not confuse genetic variation with discontinuous variation. Genetic differences can contribute to continuous characteristics as well as discontinuous ones. The difference between continuous and discontinuous describes the pattern of phenotypes, not whether genes are involved.

3:53Continuous variation involves genes and the environment, whereas discontinuous variation is usually caused by genes only.

Investigating variation patterns

4:00How could we investigate these patterns instead of just naming them? For continuous variation, we could measure leaf length in a sample of plants of the same species. We would choose a clearly defined measurement, such as the distance from the base of the leaf blade to its tip. Using the same method for every leaf makes the measurements comparable. Use a suitable ruler, record the units and avoid choosing only unusually large or unusually small leaves.

4:23Take a reasonably large, representative sample, using random selection where practical. Choosing leaves at a similar stage of development makes the comparison easier to interpret. If investigating an environmental effect specifically, keep other relevant conditions as similar as possible so that differences are not confused with unrelated influences. After measuring, organise the lengths into equal width intervals and count how many fall into each interval. A histogram is suitable because the intervals belong to a continuous measurement scale, so the bars touch.

4:54Label the horizontal axis with leaf length and its units, and the vertical axis with frequency, meaning the number of leaves. Do not assume that every continuous characteristic must produce a perfectly symmetrical bell shaped distribution. For discontinuous variation, we could examine mature pea seeds and classify each one as round or wrinkled. Instead of measuring a range of lengths, we tally how many seeds belong to each category. Use a clear classification rule and sample without deliberately selecting one shape.

5:22A bar chart is suitable because the categories are separate, so there are gaps between the bars. Investigating human blood groups requires more care because blood is a biological hazard and blood group information is personal. For this topic, use supplied, anonymous data rather than collecting blood in class. The same principle applies. Count individuals in each category and display their frequencies. There are no meaningful intermediate positions between blood group categories, so a continuous measurement scale would be inappropriate.

5:50Let us work through an example using an imaginary practice dataset, not results from a real experiment. A student classifies forty mature pea seeds and records twenty four round seeds and sixteen wrinkled seeds. First, check that the category totals account for the sample, so twenty four plus sixteen equals forty. Then ask whether the phenotypes form a range or separate categories. Here, they form two categories with no intermediates.

6:14Therefore, seed shape shows discontinuous variation, and we draw two separated bars with heights matching the frequencies. The taller round seed bar tells us that round seeds are more common in this particular sample. It does not make roundness a continuous characteristic, and it does not prove that all pea populations have those proportions. The classification depends on the possible phenotypes, not on the relative heights of the bars.

Sources of genetic variation

6:36So far, we have described differences, but where do new genetic differences originate? Mutation means genetic change. More specifically, a gene mutation is a random change in the base sequence of deoxyribonucleic acid. Bases are the chemical units whose order carries genetic information, so changing their sequence changes the information in that section of the genetic material. A useful comparison is changing a letter in a written instruction. The altered instruction may have a different effect, although a genetic change does not always produce an obvious change in phenotype.

7:07Mutation is the way in which new alleles are formed, because it can create a new version of a gene. Do not assume that every mutation is harmful, helpful or immediately visible. The word random is especially important. A mutation does not happen because an organism recognises a problem and chooses the genetic change it needs. Ionising radiation and some chemicals increase the rate of mutation, meaning that mutations occur more frequently.

7:32They do not direct the genetic material to make a particular useful allele. Mutation is not the only source of genetic variation in populations. The four sources you need are mutation, meiosis, random mating and random fertilisation. The key distinction is that mutation produces new alleles, while the other sources produce different combinations of existing alleles. Sexual reproduction brings these processes together, helping explain why offspring are not usually genetically identical to their parents or siblings.

7:59Meiosis produces gametes, the sex cells, with different combinations of alleles. You will see the detailed process in the video on meiosis. Here, the important consequence is that gametes from one parent are not necessarily genetically identical. Random mating affects which parents combine their alleles through reproduction. Random fertilisation affects which individual male gamete fuses with which female gamete, producing another source of different allele combinations. This explains why brothers and sisters can differ even when they have the same parents.

8:29Each may develop from a different combination of parental gametes and therefore inherit a different combination of alleles. They may also experience different environmental conditions, affecting characteristics such as body mass. Genetic variation and environmental variation can therefore both contribute to differences within a family. Now consider an exam style question. Describe the difference between continuous and discontinuous variation, giving an example of each. The command word describe asks you to state the relevant characteristics clearly.

8:56Begin with the pattern of phenotypes, then give examples, and include causes if the question asks for them. Do not rely only on the graph shape, because the biological distinction is the presence or absence of intermediate phenotypes. Continuous variation results in a range of phenotypes between two extremes. Body length is an example of continuous variation. Discontinuous variation results in a limited number of phenotypes with no intermediates. Seed shape in peas is an example of discontinuous variation.

9:24Continuous variation is caused by both genes and the environment, while discontinuous variation is usually caused by genes only. A second question might ask you to state the sources of genetic variation in populations. A precise answer is that mutation, meiosis, random mating and random fertilisation are sources of genetic variation. If asked which process forms new alleles, the answer is mutation, not meiosis. Meiosis changes allele combinations. It is not the process that normally creates new versions of genes.

9:55Let us finish by connecting the ideas. Variation means differences between individuals of the same species, and phenotypes may form either a continuous range or distinct discontinuous categories. Genes and the environment cause continuous variation, while discontinuous variation is usually caused by genes only. Mutation creates new alleles, and meiosis, random mating and random fertilisation create different allele combinations, helping explain why individuals within a species differ.